Devices employing a field responsive material for damping and controlling vibration and shock are known. Such a field responsive material may comprise a suitable magnetorheological (MR) material well known to one skilled in the art. As the description proceeds, the field responsive material may be referred to as either “MR medium” or “MR material” or “field responsive material” or “field controllable material”. Additionally, for purposes of clarity throughout this disclosure, devices employing such a field controllable material will generally be referred to as either “magnetorheological devices” or “MR devices” or “field controllable devices” or “field responsive devices”. MR devices may be of the “rotary-acting” or “linear-acting” variety, and can provide variable controlled torques or forces. Known MR devices may include for example rotary brakes, rotary clutches and linear dampers.
Field controllable devices typically include a housing or chamber that contains a quantity of a magnetically controllable material, and a moveable member, such as a piston or rotor mounted for movement through the material in the housing. A magnetic field generator (a coil or permanent magnet) produces a magnetic field through one or more pole pieces for directing a magnetic flux through desired regions of the controllable material.
The field controllable material employed in MR devices is comprised of soft-magnetic or magnetizable particles dispersed within a carrier, frequently a liquid. While many current applications employ a liquid carrier, it also will be appreciated that the carrier may also comprise gaseous dispersions, for example as a powder. However the required carrier is dependent on the specific application for the MR device. Typical particles include carbonyl iron or stainless steel, and the like, having various shapes, but which are preferably spherical and have mean diameters of between about 0.1 μm to about 500 μm. The carrier materials may include hydraulic oils for example.
In operation, the field controllable material exhibits a rheology change, i.e., an increase in viscosity or resistance to shear, upon being exposed to a magnetic field. The greater the magnitude of the magnetic field passing through the field controllable material, the higher the shear stress or torque that can be achieved by the MR device. Such MR materials are readily commercially available in various formulations from Lord Corporation of Cary, N.C., and the selection of the particular MR material is typically determined by the desired application for the MR device.
MR devices, in particular MR brakes, are used whenever it is necessary to control motion, and in applications where it is desirable to control the velocity or energy dissipation in a dynamic system. This includes systems irrespective of whether the systems are driven by pneumatics, manually by an operator or by another motive force generating means. The specific application is controllable energy dissipation, in the rotary sense. Energy is removed from a dynamic system to give position and/or velocity control, or to generate a desired resistance torque.
Examples of such systems include drive-by-wire systems such as might be applied in a vehicle, fork lift, or the like. In such applications, it is desirable to maintain the function of traditional mechanical controls in a system controlled in a different manner. For example, a steering wheel may be used which implements a magnetic brake, but through the use of electronics provides signals to a motor such as a servo motor, to actuate the device to be controlled, such as steered wheels, flight control surfaces, etc. Depending on the position of the device as moved by the servo motors, it may be desirable to provide tactile feedback to the operator. Thus, when turning a wheel which incorporates an MR brake, position sensors and appropriate electronics may be implemented to provide torque feedback by actuating the field generator in the magnetic brake to affect the MR material and increase resistance to motion by a rotor in the brake. For example, such an application can be a steering wheel, to which it is attached, as to maintain a realistic “feel” for the operator, in a manner duplicating the tactile feedback of purely mechanical systems.
Often, the space allotted for the use of these devices is limited and specific applications require that the devices be maintained as small as possible, while still providing sufficient resistance to the control device. It would be desirable to provide a compact, integrated device to accommodate space limitations in specific applications.
The foregoing illustrates design criteria known to exist in present field responsive devices. Thus it is apparent that it would be advantageous to provide an alternative directed to providing a field responsive device that addresses one or more of the criteria associated with present devices. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter. There is a need for controllable brakes for controlling motion. There is a need for controllable brakes and a method of accurately and economically controlling motion. There is a need for an economically feasible method of controlling motion accurately with a brake utilizing magnetic fields. There is a need for a robust controllable brake and method of making controllable brakes with improved performance. There is a need for an economic controllable brake and method of controlling motion with a magnetic field responsive material and a magnetic field generator.